Construction of a Fractionating Column ROBERT TAFT, JR., and CALVIN A. VANDERWERF University of Kansas, Lawrence, Kansas
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NE of the most important chemical laboratory operations is the separation of compounds by dist~llation. In recent years efficient fractionating columns have become standard equipment in organic research laboratories, both academic and industrial. Although construction of efficient, dependable, inexpensive, general-purpose columns for use under both atmospheric and reduced pressures is actually a simple process, nevertheless an inexperienced worker is seriously handicapped by the lack of adequate step-bystep descriptions in the literature. The present paper describes in detail the method for the construction of a simple, inexpensive, all-purpose column which, in the authors' opinions, combines the best features of several different types previously reported, and which has been found to give excellent results. These directions should prove particularly useful to workers who do not have available the funds to purchase commercially built columns. For one who has not previously used or built a similar piece of ap-
paratus, many of the techniques described should save both time and equipment which otherwise would be lost in their discovery. The parts necessary for construction of the column are : 1. A pyrex glass column 30-35 mm. in diameter (13/16-13/s inches) or less if the column is not intended for use under greatly reduced pressures, and of desired length (the columns regularly used by the authors are 4-5 feet in length). 2. Two pyrex glass tubes, one 50 mm. in diameter, (2 inches), and the other 60 mm. (23/8inches) equal in length and several inches shorter than the column (see diagrams). 3. About 50 feet of 22 gage (B. and S.) nichrome or chrome1 resistance wire. 4. A 360' thermometer. 5 . Several feet each of and l-inch asbestos tape. 6. About '/% liter of water glass. 7. Two 3-foot lengths of standard 2-inch' magnesia pipe wrapping. 8. Three feet of metal stripping used to mount magnesia-wrapped pipes. 9. A VariacZor Varitran3 transformer for control of the temperature of the nichrome-wrapped jacket. 10. Packing sufficient to fill tfie column (berl saddles' or case-hardened, triple-turn glass helices are recommended for distillations conducted under low pressures; single-turn glass3 or stainless steel3 helices, one-eighth the diameter of the column, for work a t atmospheric or somewhat reduced pressures). 11. Two ordinary clamp3 and clamp holders and an iron iing stand of sufficient height to mount the column. Precise control of the temperature of the distilling pot may be assured by use of a Glas-Col heating mantle3with an additional transformer. DIRECTIONS FOR ASSEMBLING COLUMN
Figure 1. A column of the type shown, 30-35 mm. So called because it is regularly used to cover pipe of 2-inch inner diameter; actual inner diameter of the wrapping is just under 2 1 / 1 inches. Obtainable from General Radio Company, 275 Massachusetts Ave., Cambridge, Mass. SObtainable from Scientific Glass A. ~.~ a r a t u Company. s Bloomfield, N. J. Wbtainable from Maurice Knight, Kelly Ave., Akron, Ohio. ~
(13/lrlS/s inches) in diameter and equipped with ground-glass joints, is used. The perforated glass bulb serves to hold the packing in the column and to prevent flooding. A 360' thermometer is mounted midway on the outside of the column by means of '/?-inch asbestos tape cemented with water glass. If the column is to be used a t temperatures in excess of 360°, a thermocouple may be substituted for the thermometer. Figure 2. One-inch asbestos tape is wound around the ends of the column until a wrapping of the same diameter as the inner diameter of the inner glass jacket is obtained. Water glass is applied generously to the asbestos tape, which then dries to give a rigid structure. The wrappings are spaced a t each end of the column so that, if the two glass jackets are of proper length, exactly '/? inch of the wrapping protrudes from each end when the jackets are slipped into place. Figure 3. The 50-mm. (2-inch) pyrex tube which is to serve as the inner glass jacket is cut to proper length for assembly. Two holes are blown or pulled out, one a t each end. The jacket is then fitted with two strips of '/Znch asbestos tape, which are cemented on opposite sides of the jacket over its entire length and serve to hold the resistance w,ue in place. The wire (as previously specified) is now wound about this jacket. Proper spacing of the turns is determined by the temperature range a t which the column is to be operated. One-half inch spacings are satisfactory for temperatures up to 500'. Figure 4 . Winding of the resistance wire about the first glass jacket is accomplished simply with uniform spacing as follows: Both ends of the tube are fitted with rubber stoppers through which large cork borers have been thrust about halfway. The protruding
ends of the cork borers are placed loosely in clamps supported by a pair of ring stands (4a). With such an arrangement, the wire may be wound about the jacket by rotation of the tube and spaced uniformly as desired with each revolution (4b). Figure 5. The first glass jacket is slipped into place over the asbestos wrapping. To facilitate this process, the wrapping a t the lower end of the column should fit loosely and should be left loose to give a small degree of leeway to the column. This greatly decreases the likelihopd that the lower ground glass joint of the column will be snapped off while the column is in use. The upper wrapping should fit snugly to give ample support. The two ends of the wire are run through the holes in the ends of the glass jacket and out between the asbestos wrapping and the first glass jacket. Figure 6. The protruding-asbestos wrapping ('/z inch a t both ends) is built up to the outer diameter of the glass jacket by wrapping with '/?-inch asbestos tape. Figure 7 . The ends of the first glass jacket ('/z inch of glass jacket and '/% inch of protruding asbestos tape) are wrapped with 1-inch asbestos tape until a wrapping of the same diameter as the inner diameter of the outer glass jacket is obtained. Figure 8. The outer glass jacket is slipped into place over the asbestos wrapping; it should fit snugly a t both ends. Figure 9. The protruding asbestos wrapping ( ' / a inch) a t both ends of the outer glass jacket is built up with '/2-inch asbestos tape to a diameter slightly greater than the outer diameter of this jacket so that the tape
semblies; its removal, although possible, is timeconsuming, and for this reason specially designed metal supports such as described by Todd6 are recommended for columns which are to be dismantled frequently. Any of the various types of total - condensation, variable take-off heads6 may be used in conjunction with the column. The authors prefer the type illustrated (Figure 10). It is simple, compact, and inexpensive, and may be used without auxiliary attachments for distillation a t atmospheric or reduced pressures. Hold-up of a carefully constructed still head of this type is less than I ml. An additional resistance-wire heating unit for the head may be necessary for FIounE 10. high-boiling materials. Because of the relatively large diameter of the column and the ease with which adiabatic conditions may be insured, vacuum distillations may be conducted a t pressures of less than 1mm. without flooding. Normal hold-up in a 5-foot, 35-mm. diameter column, packed with berl saddles, is less than 15 ml. For high-temperature distillations the mounted thermometer may be replaced by a chromel-almnel thermocouple. Such an arrangement has the additional advantage that t h e column may 6e converted to a dehydration furnace simply by replacement of the packing with alumina pellets. 12-15 hours are required for an inner jacket a t 300' to cool to room temperature. Packed with 5-mm. single-turn helices and equipped with astill head of the typeshown,a6-foot 35-mm. diameter column gave a fractionating efficiency of 24 theoretical plates (2.5 H.E.T.P. inches) when tested with a mixture of carbon tetrachloride and benzene a t a refluxratio of 1:30. This efficiency may be increased by division of the column heating unit into three sections so that the temperature a t the top, middle, and bottom can be controlled separately and made to agree with that a t corresponding levels on the inside of the column. Reduction in the diameter of the packed column, which also increases the efficiency, is recommended if the column is not intended for use a t greatly reduced pressures, but may cause difficulty due to flooding a t very low pressures. The entire column (including packing and a Variac transformer for control of the column temperature) may be constructed a t an approximate cost of $40. The authors wish to acknowledge their appreciation for the helpful suggestions of Robert R. Russell.
will fit snugly in the 2-inch magnesia pipe wrapping. Figure 10. Magnesia pipe wrapping (2-inch) is slipped over the outer jacket. Two 3-foot lengths of this wrapping are cut to proper length and connected by cementing together the cloth ends of the wrapping with water glass. A 4-inch wide strip of convenient length is cut from the magnesia wrapping to serve as a window for reading the thermometer and observing column operation. The removed strip, if treated generously with water glass and wrapped with cloth, maintains its shape and may be used as a door for the opening. For mounting the finished column 8n a ring stand, the following clamps have been found completely satisfactory: an ordinary clamp is disassembled to give a rod with a bolt fitting. A proper length of metal mounting stripping is fashioned so that i t fits uniformly and snugly about the magnesia wrapping (with the column within). A bolt is placed through the proper holes in the metal stripping and the hole in the rod, and the stripping is bolted tightly (so that the column does not slip) in the proper position about the magnesia with washers and nuts. One of these clamps a t each end gives sufficientsupport for the column. The authors have constructed a number of columns as described and have found the asbestos tape-water glass wrapping highly satisfactory. It is easily made and shaped, and sets to a tough binding in about 24 hours. The wrapping does not break away from the pyrex glass jackets when subjected to high temperatures, and columns constructed by its use have proved Todd, F., Ind. Eng. Chem., Anal. Ed., 17, 175 (1945). to be free from breaka~e - when treated with ordinary "orton, A. A.,'zaboratory Technique in Organic Chemistry," care. It is intended especially for permanent as- McGraw-Hill Book Company, Inc., New York, 1938, pp. 83-84.
